1. Field of the Invention
The invention relates to an internal gear pump used in, for example, an automatic transmission, or a continuously variable transmission.
2. Description of the Related Art
In an internal gear pump, a rotor installation chamber is defined by a pump body and a pump cover. In the rotor installation chamber, there are disposed an outer rotor (referred also to as “driven gear”) having a plurality of internal teeth and an inner rotor (referred also to as “drive gear”) having external teeth that define gear chambers in cooperation with the internal teeth and that are driven to be rotated while being engaged with the internal teeth. There is a known gear pump having a structure in which a suction port groove that forms a suction port and a discharge port groove that forms a discharge port are formed in at least one of a pump body and a pump cover. In this kind of gear pump, when an inner rotor is driven to be rotated at a high speed, the amount of oil sucked into the gear chambers, which are defined between the internal teeth and the external teeth, from the suction port groove is likely to be insufficient. Due to insufficient suction of the oil into the gear chambers defined between the internal teeth and the external teeth, cavitation occurs, and thus the discharge amount of oil may decrease or hydraulic vibrations, abnormal noise or the like may occur. In order to suppress occurrence of cavitation in gear chambers of a gear pump, for example, a gear pump described in Japanese Patent Application Publication No. 2005-76542 (JP 2005-76542 A) may be adopted. In this gear pump, steps extending in the rotation direction of a pump gear are formed at the bottoms of suction ports formed respectively in a pump body and a pump cover that accommodate the pump gear. The depth of a portion of the bottom of each suction port, the portion being located radially inward of the step, is set larger than the depth of the remaining portion of the bottom of each suction port, the remaining portion being located radially outward of the step.
In the gear pump described in JP 2005-76542 A, although air bubbles in the sucked oil are collected in a radially inner side portion of the suction port, part of the air bubbles are accumulated in gear chambers defined between internal teeth and external teeth. Then, the oil that contains air bubbles is discharged to a discharge port. Thus, the discharge amount of oil may decrease.